Cross-talk measuring and correcting apparatus



gfl- Another object of this invention is to provide In making cross-talk measurements on tele "r tics tame 'sra'res CROSS-TALK MEASURING AND CORRECT ING APPARATUS Lester Hochgraf, MaplewoodQN. L, assigncr to Bell Telephone Laboratories, Incorporated; New York, N. Y, a corporation oi New York Application December a 1937, Serial No. 178,338 iz'olaims. -(o1. Pia-175.3)

This invention relates to cross-induction beout of M. A. Weaver, 2, issued May 11, tween transmission lines and particularly to 1937- means for the ready evaluation and correction The value of far-end cross-talk between two of such cross-induction or cross-talk. pairs A and B where pair A is the disturbing cir- Such evaluation and correction of cross-talk cult, is usually somewhat different from the has particular importance between telephone ca value where pair B is t disturber v though ble pairs used for carrier current transmission. pairs A and B have substantially the same However, it has considerable significance in ccn-- attenuation and characteristics. These two nection with many other transmission systems. values are called the AB and BA cross-talks m 1 An object of this invention is to provide means respec ively i for the easy and rapid determination of the A two-way Co p balancing network y ture and magnitude of the corrective means rebe used to balance the average of these two crossquired for the correction or elimination of crosstalk values. Compromise balancing can be cartalk between two pairs-of transmission lines. ried out as follows:

Another object is to provide means for the 1 Measmethe AB'croSs ta1k 15 rapid and easy determination oithe average Measure-the BA 01.05549 cross-talk between two pairsof transmission lines Compute the average of these two values f fi Q and thenfhefnher 9 the two palm 4. Adjust the balancing network to balance this Or lines is made the disturbing line. average giving equal AB and residua15 0 7 While this method is fairly satisfactory for means whereby the evaluation of the cross-talk balancing at a single frequency, it is a rather between two pairs of transmission lines is made it ti th requiring a minimum slow process. For that reason it is not satisfacpenditure of time and effort on the part of the W if balanCing is be 610m at a Bl 0f operator. u I frequencies or over a,- band of frequenmes. Ihe 25 A th bje t is t provide means for the disclosure of my invention presents a satisfactory ready determination of the average cross-talk methodfor fafjmating compromls? balancing between two pairs of transmission lines when h a smgle frequency, mu1 t1"frequency transmitting any'simple or complex Waveand r type speclfic 317: also when transmitting a warble tone. of practlce accordmg to my m I Angularv object is to provide means for the at one end of the lead or cable section of which rapid adjustment of an adjustable network the two lines or pairs form a part, an oscillator adapted to correct cross-talkbetween two pairs or other sourfze of testmg current at w P- f lines. end an amplifier-detector; at some th1rd point which may well be the center but might be any other point between the two line terminals, a

line balancing device known as an admittance unbalance bridge is connected. A slow-acting or integrating meter which may be a milliammeter is located at this third point and wired through some form of detector to the other line called to the Output amplifier'detector' A urernent. or setting may be made by connecting the disturbed line, the oscillator and the detector beinl located t the Same end of mm m tne oscillator to one palr, the amplifier-detector f M g Y i m r t to'the other pair and adjusting the admittance cmsswa measu emen e oscla'ior unbalance bridge until the milliammeter readphone lines, one ordinarily distinguishes between near-end cross-talk and far-end cross-talk. Near-end cross tall measurements are ordinar ily made by connecting 'an oscillator to one pair 40 of wires or line called the disturbing line and 45- l D I IS located at one endv of the line and the d..tectoi mg is a minimum The bridge reading then at the Other h m m dicates the dimensions of the balancing network by Several hundred e My invent-Q1115 P required to reduce the cross-talk to its minimum ticularly useful in providing means to facilitate the determination and correction of far -end I may also provide means forrapidl 50 Cross-$311k- This Correction is ordinalfly achieved ing the oscillator and the amplifier-detector back by the addition or insertion of resistive reand forth between the two pairs, taking care active circuit elements after the manner, for to so synchronize the'switching that the oscilexample, disclosed in the patent of A. G, Chaplator and amplifier-detector are always on a1 I man, 1,863,651, issued June 21, 1932, or the patternate pairs. Under this condition the reading switches I and I6.

of the meter attached to the output of the am plifier-detector will indicate the AB and the BA cross-talk alternately and if the switching is rapid enough or the action of the meter properly damped the average of the AB and the BA cross-talk will be indicated. -When the admittance unbalance bridge is so set as to reduce this average meter reading to a minimum the dimensions of the compromise balancing network will be indicated by the bridge readings.

It will be understood that this disclosure has relation to a specific example of my invention and that the scope of the invention is not limited by it but is defined in the appended claims.

Referring to the drawing:

Fig. l is a diagram presenting in schematic form an arrangement according to the invention for determining the far-end cross-talk between two lines and for determining at the same time the dimensions of the balancing network required to reduce the cross-talk between said two lines to a minimum value; and

Fig. 2 is a schematic drawing illustrating a method of synchronizing the commutator for operating the electromagnetic switches, with the air condenser in the warble tone oscillator.

Fig. 1 shows two lines A and B subject to crosstalk the one with the other. At one end of these lines an oscillator l! is connected through the blades I? and i3 of the electromagnetic switch l5 to the line A and a line terminating network I3 is connected through the blades l9 and 26 of the electromagnetic switch l5 to the line B. These connections may be said to be at the far end of the line. At the other or near end of the line, a terminating network 54 is connected through the blades 2| and 22 of the electromag netic switch l6 to line A and an amplifier-detector 25 is connected through blades 23 and 24 of electromagnetic switch IE to line B. At some suitable and convenient intermediate point a balancing device which may be an admittance unbalance bridge 65 is connected between lines A and B. Also at this intermediate point is provided a meter 34 which is wired to the amplifierdetector 25 and by its readings indicates the amount of current picked up by the amplifierdetector. The condition shown is that in which the electromagnetic switches I5 and I6 are energized or operated and the apparatus is in the position to measure the AB cross-talk, that is, the cross-talk from line A to line B. If the switches I5 and iii are deenergized then terminating network E3 is connected through blades II and 3 to line A and oscillator H is connected through blades I 9 and 26 to line B at the far end. At the near-end terminating network I4 is connected through blades 23 and 24 to line B and amplifier detector 25 is connected through blades 2! and 22 to line A. Under this condition the apparatus is in the arrangement to measure the cross-talk between line B and line A, that is, the BA cross-talk. In order that the equipment may be switched synchronously or simultaneously from the AB position to the BA position and, further, in order that this switching may be accomplished at any desired speed, there is provided a commutator 26 through which a grounded battery 29 provides operating current for The operating current for switch 25 may be traced from ground to battery 29, commutator 28, brush 36, lead 36 and lead 2! to the electromagnetic switch l5 and thence to groiuid. The operating current for switch [6 flows from ground to battery 29, commutator 28,

brush 36, lead 36, lead 32, thence through phantom coil 26 on spare line C to the near end phantom coil 21, lead 33 and the winding of the electromagnetic switch I6 and thence to ground.

In Fig. 2 a motor 44 drives a shaft having live portions 41, 46 and 45 and insulating portions 48. Mounted on the live portion 46 of this shaft is the commutator 28 and on the live portion 45 an air condenser 55 having two fixed plates 3'. and 33 and a movable plate 39. This air condenser 55 is connected into the tuning circuit of oscillator H as indicated by dotted lines in Fig. 1 in case a warble tone is required for testing. A warble tone is a tone whose frequency varies cyclically through a predetermined frequency range. It may be produced for example from a vacuum tube oscillator having a motor driven air condenser in its tuning circuit as described for example in Arnold et a1. Patent 1,573,367 issued February 16, 1926.

When the condenser 55 is in the position shown in Fig. 2, the capacity between the fixed plates 31 and 38 and the movable plate 39 is at a mininorm and consequently the frequency of the oscillator is at its maximum. When the shaft has been rotated through 90 degrees from the position shown in Fig. 2 the capacity between the fixed plates and the movable plate is at its maximum and the frequency of the oscillator at its minimum or lowest frequency. It will be apparent, therefore, from the fixed relation between the commutator and the air condenser that the oscillator will pass through a complete frequency cycle for each position of the switches l5 and E6 of Fig. 1.

The use of the admittance unbalance bridge at 35, Fig. 1, as an instrument for the determination of the measure of cross-talk, or conversely for the determination of the dimensions of the compensatory or balancing network required to compensate against cross-talk is well known. Discussions of its use are to be found, for example, in the patent to Chapman 1,863,651 issued June 21, 1932.

In the determination of the balancing network in accordance with my invention, the oscillator II of Fig. 1 is adjusted to give the desired testing current. This may be a warble tone in which case the oscillator frequency will be set at some suitable frequency by means of its tuning adjustment, usually by adjusting the value of the capacity in the frequency determining circuit of the oscillator, and the auxiliary condenser 55 connected in parallel with this oscillator tuning condenser. When the shaft 45 is rotated the frequency of the oscillator will vary through a cycle. By properly proportioning the relation between the oscillator tuning condenser and the auxiliary condenser 55, the desired allocation and width of band may be produced.

Having set the oscillator H to deliver the desired testing current, the motor 44 may be energized causing (a) the commutator to revolve thus alternately energizing and deenergizing the switches l5 and I 6 by means of the battery 29, and (b) varying the capacity of the condenser 55. thus synchronously causing the frequency of the oscillator to pass through its cycle or band of frequencies twice for each revolution of the drive shafts 45, 46, 61, 43 and thus impressing the complete cycle of frequency alternately on line A and line B. If the oscillator is set up to deliver a single frequency only, then the condenser 55 will not be connected into the oscillator tube circuit and the single frequency will be imales-n40;

pressed for the period of. one-half revolution of the motor shafts 45, 46,41, 48 alternately on line A and line B.

While these adjustments andconnecti'onsare being made at. the far end of the line, the and-- plificr-detector at the near' end will have been energized and its output connected through tothe meter t llocated beside the admittance unbalance bridge 35; Assuming that the bridge is "Shforthis condition I am ableto read' oli" the set initially for zero compensation, that'is, so

that there would be no indicated cross-talk if" the lineswere perfectly'compensated, then, withthe bridge in this condition, the meter a l-should ond and I employ a meter 34} having a movingsyster'n so damped or ofsuch a natural period that at the speed of commutation from the AB condition to the BA condition and alternately back and forth the needle ofthemeter does not flicker up and down but assumes-asubstantially steady fixed position representative for the average between AB cross-talk and BA cross-talk. Having the far-end and near-end apparatus operating properly as described, I now adjust the admittance unbalance bridge 35 to the position giving zero or minimum reading at meter 3 5 and from. the setting of the several arms of the bridge dimensions of the compromise network to compensate for the existing cross-talkafter the manner isclosed in Chapman Patent 1,863,651.

In place of'the calibrated admittance unbalance bridge, I may employan adjustable network.

which when adjusted for minimum cross-talk accordingv to .the above: procedure may remain as a permanent line balancing and cross-talk preventing network. I i

It will be apparent that ordinarily at least three operators will be required in making cross-talk corrections according to my invention, namely, an

operator at the far end, an operator at the near end, and a third to operate the admittance unbalance bridge. However, the same number oi. operators have ordinarily been required in the determination of far-end. cross-talk and compromise networks under the conditions previously employed, whereas when my invention is used the time required for making the determination is greatly reduced.

Although my invention has been disclosed in terms of a particular embodiment, it must not be considered limited by these terms but rather by the following claims.

What is claimed is:

1. In combination, a pair of circuits, at source of oscillations, a measuring device and means for continuously switching said source between said circuits in alternation with said measuring device so that said source and said measuring device are each constantly connected to one of said pair of circuits but not simultaneously to the same circuit.

In combination, a pair of adjacent signaling circuits, a source of waves of a test frequency, cross-talk measuring means, a first switching means operating to connect said source in rapid succession alternately to each of said c rcuits at adjacent points so as to transmit pulses of said test frequency successively over said circuits, a

I second switching means operating in synchronism with said first switching means to connect said measuring means alternately to said circuits and inalternation with said first switching means so that it will receive the cross-talk pulse due to each pulse of test frequency transmitted over said circuits,- the rate of switching-being such that the measuring means'will indicate the average value of cross-talkbetween said circuits.

3. In combination, a pair of signaling: circuits adjacent-and parallel to each other; a source of alternatingcurrent; cross-talk: detecting means; a-"firstswitching means for connecting the circuits-in rapid rotationto said source; asecond switching means for 'connectingthe circuits in rapid rotation to said detecting means, meansfor so-synchronizing said first and second switching means that said source and said detecting means are at no-time connected to the-same circult, and balancing means for balancing out: cross-talk between said circuits.

4: Inansystem for making measurements of' cross-induction between two lines or pairs of conductors subject to-cross-induction either one into th'e-other, means at-the far-end for producing a testing current ofany desired characteristic,- means at the near end for measuring. any smallfraction of saidtesting current which may be receivedl there, means for connecting said source of testing current alternately to first one and then the other of said two lines, coacting means for simultaneously connectingv the said switching said testing current source to one of said lines and. said current detecting means to the other of said lines, mean associated with said switching means for rapidly interchanging the connections of. said testing current source and said current detecting means respectively between said two lines, balancing means connected between said two pairs of lines adapted to reduce the cross-talk between said two lines, and suitable means associated with said current detecting means for indicating a minimum current.

6. A system according to claim 5 in which the balancing means connected between said two pairs of lines consists of a calibrated admittance unbalance bridge such that its readings indicate the dimensions of a balancing network equivalent to thebridge in cross-talk reduction.

7. A system according to claim '5 in which said balancing means consists of a calibrated admittance unbalance bridge with which is associated a damped electric meter operated from said current detecting means and adapted to indicate the average cross-talk between the two lines.

8. A system for effecting cross-talk reduction between two lines comprising means for producing a suitable source of testing current first on the one or A line and then on the other or B line, means for detecting alternately the current induced in the 33 line by the testing current in the A line, and then the current induced in the A line by the testing current in the B line, means for rapidly switching the testing current from line A to line B to line A, and so forth, means for simultaneously switching the current detecting means from line B to line A to line B, and so forth, minimum current indicating means associated with said current detecting means adapted to give a steady reading averaging the current into the said detecting means during the said switching, and means associated with said lines adapted to reduce said steady reading to a minimum.

9. In combination, a pair of transmission lines A and B running between two terminals or reference points in the same pole line or in the same cable, at the far end of said line means for producing suitable testing current, a line terminating network suitable for terminating either line A or line B at the far end, and a remotely controlled two-position switch which in its operated position may connect said means for producing testing current to line A and said terminating network to line B and in its nonoperated position may correspondingly connect said means for producing testing current to line B and said terminating network to line A; at the near end of said line a line terminating network suitable for terminating either line A or line B at the near end, current detecting means suitable for detecting suitably small amounts of the species of current used in testing, and a remotely controlled two-position switch which correspondingly to the far end switch may in its operated position connect said near end terminating network to line A and said current detecting means to line B, and in its non-operated position may connect said current detecting means to line A and said near end terminating network to line B; at a third point on said line an admittance unbalance bridge connected between said lines, and a suitable device connected through from the said current detecting means and capable of indicating the magnitude of the current picked up by the said current detecting means, and particularly to precise minimum of said current; and a motor-driven commutator for periodically operating and releasing said remotely controlled switches at any desired rate.

10. The combination of claim 9 in which the source of testing current consists of a vacuum tube oscillator, the frequency of which is variable over a predetermined band by means of a variable air condenser in the frequency determining circuit of the said oscillator, the said air condenser being so coupled with the means for operating the remotely controlled switches that the operation of the switches is synchronized with the change in frequency of the oscillator and identical wave trains are sent out On lines A and B alternately.

11.. In combination, two transmission lines A and B subject to cross-talk either one with the other, means including a damped indicating meter for rapidly determining and balancing out the AB cross-talk and the BA cross-talk, and means for determining the compromise network for maxinmm freedom from both AB and BA crosstalk.

12. A cross-talk measuring and balancing set comprising a set of line terminating networks, a vacuum tube oscillator equipped to produce single frequency or warble tone output, an amplifierdetector with damped indicating meter,- switches for simultaneously switching between oscillator and network at the one end of the line and between network and detector amplifier at the other end of the line, an admittance unbalance bridge, and a combined commutator and condenser drive for operating the switches and the warble tone oscillator in synchronism.

LESTER HOCHGRAF. 

